In the design of central processing units (CPUs) used in modern computing systems, improvements must take into account the command structure and the word length which often must remain the same as used in the predecessors to a "new" CPU. This is particularly true in the case of systems running under proprietary software operating systems. The fundamental reason for this constraint is the often enormous investment in software which a user may have. In some instances, there may be represented three decades of software development in a given user system, and a user typically wishes to merely load its old software on a new, improved hardware system and experience immediate higher performance results. As a result, CPU designers go to great lengths to maintain compatibility with earlier CPUs in a given operating system family.
Thus, in CPU design based on existing systems, even limited modification of the address range capability (i.e., allocation of address fields in a CPU instruction word) of each CPU would have a major impact on hardware design and, more especially, on the software which can or must be reused. This dilemma is faced in view of the ongoing increase in density and capacity of memory chips and other technologies that have resulted in the availability of mass memories capable of storing information which is one to two times greater than was contemplated at the time the address field lengths/word lengths were originally established in the operating system/CPU family.
Another factor involved in the design of CPUs, particularly CPUs intended to function under a long-established proprietary operating system, is the fact that ever-new technology allows an existing CPU design to be physically implemented into a much smaller space. However, again, the advantages of adhering to the fundamental specifications of existing operating system software (in order to permit the reuse of existing application software) directs the retention of the operating system, nothwithstanding its limiting effects on CPU design. Finally, and perhaps most importantly, those skilled in the art are aware of inefficiencies which occur when too many CPUs are operated in a single operating system environment.
All these factor are aggravated in a multiprocessor cluster environment in which multiple groups of multiple CPUs are incorporated into a large computer system which includes a large main memory containing stored information at more addresses than can be directly addressed by the individual CPUs, with the addressing constraint being imposed because of the address field/word length characteristics which are the natural heritage of the historical hardware and operating system/application software of a given computer system family. It is to the solution of this dilemma that the subject invention is directed.